Analysis method employing ionization potential of metastable krypton atoms for detecting unsaturated compounds



R. s. SIILAS UNSATURATED COMPOUNDS Filed May 24, 1965 Aug. 19, 1969 ANALYSIS METHOD EMPLOYING IONIZATION POTENTIAL OF METASTABLE'KRYPTON ATOMS FOR DETECTING INVENTOR R. Sv SILAS A TTORNEVS United States U.S. Cl. 7323.1 8 Claims ABSTRACT @F THE DXSCLUSURE A process for analyzing a vaporous mixture of saturated and unsaturated organic compounds for the presence of said unsaturated compounds comprising passing the mixture through an ionization detection zone in the presence of krypton, forming metastable krypton atoms by radiation Within said zone, which atoms ionize only the unsaturated molecules having ionization potentials below 10 electron volts, and transmitting from said zone an electrical signal representative of the concentration of unsaturated compounds in said mixture. In a second modification the mixture with krypton as a carrier gas is run through a chromatographic zone before entering the ionization detection zone, so that the different molecular weight unsaturated compounds may be spaced in time of arrival at the detection zone by their differential time of passage through the chromatographic zone and thereby further identified. A third modification of the invention differs from the second modification by using in place of krypton an inert diatomic carrier gas, such as hydrogen, nitrogen or carbon dioxide, which does not form relatively long-lived mestastable atoms on being exposed to radiation (as krypton does). This diatomic carrier gas carries the hydrocarbon mixture through the chromatographic zone and then krypton is added before passing the efiiuent from the chromatographic zone into the ionization detection zone.

This invention relates to a method and apparatus for the analysis of hydrocarbon mixtures. In another aspect, this invention relates to a method and apparatus for the elective analysis of hydrocarbon mixtures.

In the analysis of hydrocarbon mixtures, it is oftentimes desired to determine, for example, the presence of unsaturated hydrocarbons in a mixture of saturated and unsaturated hydrocarbons. Under such circumstances, it would be advantageous to employ a method of analysis wherein the unsaturated constituents of the hydrocarbon mixture can be isolated or selected for analysis.

Accordingly, an object of my invention is to provide an improved method and apparatus for analysis of hydrocarbon mixtures.

Another object of my invention is to provide an improved method and apparatus for the analysis of organic constituents wherein the molecules of the constituents have ionization potentials below 10 electron volts.

Another object of my invention is to provide an analysis method and apparatus for the detection of unsaturated organic compounds.

atent 3,4522% l Patented Aug. 19, 1969 Another object of my invention is to provide an improved chromatographic method and apparatus for the analysis of hydrocarbon mixtures containing unsaturated compounds.

Other objects, advantages and features of my invention will be readily apparent to those skilled in the art from the following description, the drawing and appended claims.

By my invention, I pass a vaporous organic mixture in combination with krypton to an ionization detector. Within the detector, relatively long-lived metastable krypton atoms are formed by radiation from a beta source such as tritium or other radioactive isotopes. The metastable atoms, on collision with vapor molecules, transfer their excess energy to the molecules. The metastable energy level of krypton is 10 electron volts; thus, only molecules with ionization potentials below 10 electron volts will be etected.

For organic compounds, 10 electron volts is the ionization potential dividing point between saturated compounds whose ionization potentials are greater than 10 electron volts, and unsaturated compounds whose ionization potentials are less than 10 electron volts. The invention is applicable to, for example, the detection and measurement of aromatic hydrocarbons in petroleum fractions employing gas chromatographic methods of analysis. Such petroleum fractions generally contain components, many of which are incompletely separated and many of which are saturated. The invention permits measurement of the aromatic components without interference of the remaining components of the mixture.

The invention is applicable to the detection of trace components of unsaturated hydrocarbon compounds in hydrocarbon mixtures. An example would be the presence of trace components of aromatic hydrocarbons in normal parafiins. A sample mixture could be vaporized and then sent through the ionization detector with no attempt at separation in a chromatographic column, resulting in an electrical signal transmitted by the ionization detector proportional to the amount of aromatics present in the hydrocarbon mixture. Other applications of the invention include detection of aromatics and olefins in polymerization grade solvents such as hexane, pentane and cyclohexane.

The drawing is a schematic representation of the first, second and third embodiments of the invention.

Referring to the drawing, in a first embodiment of the invention, an inert diatomic carrier gas such as hydrogen, nitrogen or carbon dioxide is passed from carrier gas storage vessel 10 via conduit means 11 to a conventional drying zone 12. From drying zone 12 the dried carrier gas is passed via conduit means 13 to a conventional sampling valve 14 wherein the carrier gas is mixed with a vapor sample introduced into sample valve 14 via conduit means 16. The sample mixture introduced into sample valve 14 is a vaporized mixture of organic compounds.

The sample and carrier gas vaporized mixture is passed via conduit means 15 from sample valve means 14 to a chromatographic column 17 containing a partitioning material capable of selectively retarding the flow of the constituents of the sample mixture directed thereto. An effluent mixture is withdrawn from the top of chromatographic column 17 via conduit means 18 and passed to a conventional ionization detector 21.

Detector 21 is a conventional ionization detector such as a Beta-Ionization detector manufactured by MicroTek Instruments, Inc., 5500 Oak Villa Blvd, Baton Rouge 15, La. Ionization detector 21 contains a conventional beta radioactive source such as tritium.

Krypton is introduced into the column 17 efiluent mixture via conduit means 19 and valve means 20. Within ionization detector 21 relatively long-lived metastable krypton atoms are formed which on collision with the vapor molecules of the sample mixture transfer their excess energy to the sample vapor molecules. As the metastable energy level of krypton is electron volts, only the vapor molecules having ionization potentials below 10 electron volts will be detected within the ionization detector 21 as indicated by a DC electrical signal transmitted from ionization detector 21 via conduit means 22.

The efiluent mixture is vented from ionization detector 21 via vent means 27. An electrical signal representative of the concentration of unsaturated compounds or compounds having an ionization potential below 10 electron volts is transmitted from ionization detector 21 via conduit means 22 to a conventional electrometer 23 wherein the received signal is amplified and the amplified signal transmitted via conduit means 24 to a conventional recorder 26.

In a second embodiment of the invention krypton can be used as a carrier gas eliminating the necessity of injecting krypton into the efiluent mixture withdrawn from chromatographic column 17 via conduit means 18.

In a third embodiment of the invention wherein, for example, it is desired to obtain the concentration of trace unsaturated constituents of an organic mixture; the organic sample mixture can be passed via conduit means 16, conduit means 28, valve means 29 and conduit means 18 directly to ionization detector 21. In this embodiment of the invention, krypton is mixed with the sample within conduit 18 by the passage of krypton via conduit means 19 and valve means 20 to conduit means 18.

The following example is presented to illustrate the objects and advantages of my invention. The particular effectiveness of krypton in the selective analysis of organic compounds is demonstrated by comparison with the results obtained when using argon in place of krypton.

Example The chromatographic column employed in this example was a 4' x As" O.D. glass column packed with a mixture of 2 weight percent SE 30 silicon rubber on 70/80 mesh chromosorb G, AW-BMCS (produced and distributed by Johns-Manville). The ionization detector employed was a MicroTek GC 2500R triode detector equipped with an ionization source comprising tritium. Polarizing voltage for the ionization detector was supplied from a MicroTek power unit and was set at 1,000 volts DC. The chromatographic column was operated at the optimum temperature for each sample component, with the ionization detector and the outlet block maintained at 170 C. The carrier gas flow was approximately 40 cc./minute.

In this example, argon and krypton were alternately employed as the carrier gas for organic compound samples, both hydrocarbons and non-hydrocarbons, with the results of the runs presented below in the table. A 10 microliter liquid sample in each run was injected into the carrier gas stream and the resultant vaporous mixture passed to the above-described chromatographi column and detector.

Referring to the table below, the absolute sensitivity for each compound with argon and krypton is measured in mols/ second. Sensitivity was determined by calculating the weight of sample which would produce a 5 mm. peak height. This quantity was then divided by the molecular weight of the compound and by the time taken for the TABLE.ARGON AND KRSXEEP QTICIZIIgI SENSITIVIIIES, SECOND Absolute sensitivities, Sensitivity moles/second X 10 ratio,

Krypton/ Ionization Krypton Argon potential Compound Argon n-Pen tane. n-Hexane n-Heptane. n-Octane 1,5-hexadiene 1,3-cyol0l1exadiene. n-Butyl mereaptan. Methyl ethyl sulfide Diethyl disulfide. Naphthalene Referring to the sensitivity ratio krypton/ argon of the above table, it is readily apparent that krypton is particularly effective in selectively analyzing organic mixtures. The aromatic compounds in the table illustrate at least 10 times greater sensitivity than the normal paratfins with krypton, whereas the sensitivities of the compounds are comparable with argon. A comparison of the ionization potentials clearly demonstrate that the break point is about 10 electron volts.

As will be evident to those skilled in the art, various modifications of this invention can be made, or followed, in the light of the foregoing disclosure, without departing from the spirit or scope thereof.

I claim:

1. A method for analyzing a vaporous organic mixture of saturated and unsaturated organic compounds for the presence of said unsaturated compounds which comprises passing said mixture to an ionization detection zone in combination with krypton, forming metastable krypton atoms with an energy level of 10 electron volts by radiation within said ionization detection zone, said atoms ionizing only the unsaturated compounds present in said mixture, transmitting from said ionization detection zone an electrical signal representative of the concentration of unsaturated compounds in said mixture, and withdrawing an effluent vaporous mixture from said ionization detection zone.

2. The method of claim 1 wherein said vaporous organic mixture comprises a vaporous hydrocarbon mixture.

3. A method for analyzing a vaporous organic mixture of saturated and unsaturated organic compounds for the presence of said unsaturated compounds which comprises introducing said mixture into a chromatographic zone containing a partitioning material that selectively retards the passage therethrough of the constituents of said mixture, introducing krypton into said chromatographic zone, passing an efiluent mixture from said chromatographic zone to an ionization detection zone, forming metastable krypton atoms with an energy level of 10 electron volts by radiation within said ionization detection zone, said atoms ionizing only the unsaturated compounds in said mixture, transmitting an electrical signal from said ionization detection zone representative of the concentration of unsaturated compounds in said mixture at the time of their passage therethrough, and withdrawing from said ionization detection zone an effluent vaporous mixture.

4. The method of claim 3 wherein said vaporous organic mixture comprises a vaporous hydrocarbon mixture.

5. A method for analyzing a vaporous organic mixture of saturated and unsaturated organic compounds for the presence of said unsaturated compounds which comprises introducing said mixture into a chromatographic zone containing a partitioning material that selectively retards the passage therethrough of the constituents of said mixture, passing an inert diatomic carrier gas to said chromatographic zone, passing an eflluent mixture from said chromatographic zone to an ionization detection zone, introducing krypton into said efiluent mixture, forming metastable krypton atoms with an energy level of 10 electron volts by radiation within said ionization detection zone, said atoms ionizing only the unsaturated compounds present in said mixture, transmitting an electrical signal representative of the concentration of unsaturated compounds in said mixture at the time of their passage therethrough, and withdrawing from said ionization detection zone an efliuent vaporous mixture.

6. The method of claim 5 to include amplifying electrical signal transmitting from said ionization detection zone and recording said amplified signal.

7. The method of claim 6 wherein said vaporous organic mixture comprises a vaporous hydrocarbon mix- 20 ture.

References Cited UNITED STATES PATENTS 3,009,063 11/1961 Roehrig 250-836 3,009,060 11/1961 Roehrig 250-435 3,117,225 1/1964 Willis 250-435 OTHER REFERENCES Lipsky et a1.: Chemistry," Nature, vol. 200, 4906 at pp. 566-67.

RICHARD C. QUEISSER, Primary Examiner V. I. TOTH, Assistant Examiner US. Cl. X.R. 

